1. Field of Invention
The present invention relates to an improved electrical insulating material, and, in particular to an improved electrical insulating material comprising at least one polymer of ethylene and an improved anti-tracking, anti-erosion additive.
2. Brief Description of the Prior Art
The occurrence of the phenomenon of tracking in polymeric materials used as electrical insulation in environments wherein they are exposed to contaminated or polluted atmospheres has long been recognized as a source of electrical insulation failure in such applications. This is particularly true of heat shrinkable tubing used in high voltage splices and in high voltage terminations used by utility companies. Generally speaking, this phenomenon results from the fact that the insulation above described gradually acquires a conductive film of contaminants on its surface which, when the insulation is subjected to high voltage stress, will over time allow a small amount of leakage current to conduct along the surface. The resulting temperature rise causes a drying out of the insulation. Once this has happened the dry surface areas so formed can frequently encounter electrical stress greater than the insulation air interface, thereby resulting in a spark or scintillation. The tremendous temperatures in the spark interior, often exceeding 2000.degree. C., can quickly degrade the insulation to carbonaceous material that is highly conducting. This in turn leads to additional scintillations and material degradation eventually resulting in a burned, tree-like pattern on the surface of the insulation. When the span between electrodes has been fully bridged by this pattern, failure due to creepage tracking has occurred.
Similarly, over-voltage exposure also tends to cause gradual and progressive erosion or cratering of the insulation over time. The resulting lessening of the insulation thickness causes a corresponding decrease in the insulative characteristics of the insulation in the eroded area and can lead to insulation failure if such erosion is not inhibited.
It should be understood that discharges of the creepage tracking type are not the same as those caused by the establishment of an arc between two elements of an apparatus having different potentials, and that materials which have been found to be effective in inhibiting direct arcing are not necessarily effective in inhibiting creepage tracking. This fact was disclosed by Kessel et al in U.S. Pat. No. 2,997,526 and our tests tend to confirm this distinction.
Numerous proposals are present in the art for the prevention and/or inhibiting of creepage tracking. Of these proposals, the most widely accepted has been the incorporation of hydrated alumina, preferably the trihydrate, in fairly substantial quantities into the insulating material as is disclosed for example in U.S. Pat. Nos. 2,997,526; 2,997,527; 2,997,528; and 4,100,089. It has been found, however, that the large amounts of hydrated alumina required frequently undermine the physical properties of the insulation, and that, depending on the polymer system involved, the hydrated alumina exhibits fairly widely varying degrees of effectiveness. In the case of polyolefin materials such as polyethylene and copolymers like ethylene-vinyl acetate which are widely used in the making of heat-shrinkable insulation, for example, testing has shown that alumina trihydrate provides only minimal protection against tracking. The reasons for this are not clear. Polyethylene and related polyolefins are known to be among the most track resistant of polymers. It is similarly well known that polyethylene and/or related polyolefin insulation requires the presence of additives such as anti-oxidants, lubricants, and crosslinking agents with the basic insulating material to insure the desired physical characteristics thereof. These additivies usually lower the track resistance of the insulation below that of the pure polymer. The addition of alumina trihydrate improves the track resistance of insulation composed of polyolefin and additives somewhat, but does not always even bring it to the point at which the pure resin itself tracks.
In addition, it has been found, for example in U.S. Pat. Nos. 4,100,089 and 4,223,071, that alumina trihydrate does not adequately increase erosion resistance and that therefore separate additives are required for that purpose.